November 8. 2008 05:48

Very good article! I have started using your Linq.Specifications pattern and I have hit a wall when attempting to write a QuerySpecification for a parent - child situation. Here is my Expression method so far:

public override Expression<Func<Source, bool>> MatchingCriteria
{
get { return parent => parent.Name == name && parent.Children....//what next??? }
}

Is this the right way to go about this or should I write two separate QuerySpecifications and combine them somehow?

Darren

November 8. 2008 08:09

Got it now...

return parent => parent.Name == pname && parent.Children.Any(child => child.Name == cname)

Darren

November 11. 2008 08:08

Hi! I was wondering if you have any idea what's happening here in my example. When I run the following code:

var getbyid = new ProductByIdSpecification(3);
var foundProduct = productRepository.FindOne<Product>(getbyid);

I am getting the following exception:

NHibernate.MappingException: No persister for: System.Converter`2

At first I thought it might be something to do with Linq-NHibernate so I tried this (which worked ok):

var foundProduct = (session.Linq<Product>().Where(p => p.Id == 3)).First();

So I am thinking it must be something to do with the call to Convert made in the QuerySpecification class. Am I going down the right path with this? Have you seen this problem before and, more importantly, do you have a solution at hand?

I am using Fluent NHibernate by the way.

Darren

November 11. 2008 22:31

Thanks Steven. I appreciate your time. I have spend a few hours trying to figure this out already and have had no luck so far. I just can't figure out why there should be a MappingException for the System.Converter delegate which itself should simply return the same mapped entity type of "Product".

I can provide some more details of what I have done below and as you can see it is a very simple example of "GetById" that I am trying to execute. As mentioned previously, I can get this to work using the session object directly and also by using "RetrieveAllSpecification" (probably because in these examples do not need to call the "Convert" method).

So here are some more details...

The specification I am using is a basic "GetById" example that returns the same type as the mapped entity (in this case "Product"). Here is the specification class:

public class ProductByIdSpecification : QuerySpecification<Product>
{
private readonly int id;
public ProductByIdSpecification(int id) { this.id = id; }

public override Expression<Func<Product, bool>> MatchingCriteria
{
get { return p => p.Id == id; }
}
}

This is my calling code:

var getbyid = new ProductByIdSpecification(3);
var foundProduct = productRepository.FindOne(getbyid);

And this is the code in the repository:
public TResult FindOne<TResult>(ISpecification<T, TResult> specification)
{
return FindAll(specification).First();
}

public IQueryable<TResult> FindAll<TResult>(ISpecification<T, TResult> specification)
{
return specification.SatisfyingElementsFrom(session.Linq<T>());
}

The following exception happens when calling Convert (in QuerySpecification<T, TResult> class):

NHibernate.MappingException: No persister for: System.Converter`2
[[Dadou.RepositoryPattern.Model.Product, Dadou.RepositoryPattern.Model, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null],[Dadou.RepositoryPattern.Model.Product, Dadou.RepositoryPattern.Model, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]]

Darren

November 12. 2008 20:38

Thanks Steven for the workaround which does indeed work! Thanks also for the explanation which makes it clear to me now why it was not working before.

Darren